In a next-generation power network called smart grid, a Smart Meter (hereinafter referred to as SM), which is an electricity meter for measuring power consumption of electrical equipment, is installed in each area (e.g. home) on which power consumption should be measured. The SM communicates with a Meter Data Management System (hereinafter referred to as MDMS), which is a data management device, through the power network. The MDMS collects power consumptions per unit time from the SMs installed in the respective homes etc. The power consumptions per unit time collected by the MDMS are utilized by e.g. an Energy Management System (hereinafter referred to as EMS) connected to the power network. Based on the total power consumption about a plurality of homes etc. obtained from the MDMS, the EMS controls power by requesting each home in a targeted management region to restrain power consumption, or by charging/discharging a storage battery connected to the power network. The MDMS can provide the power consumption not only to the EMS but also to a system (hereinafter collectively referred to as application server) utilizing the total power consumptions collected from the SMs. Accordingly, the MDMS retains the power consumptions collected from the SMs in order to calculate the total power consumption to be utilized by the application server later.
However, the MDMS directly retaining the power consumption measured by the SMs can be illegally obtained by an administrator of the MDMS or a user illegally entered the MDMS to guess the activities in each home, which leads to invasion of privacy. Therefore, it is under consideration that the MDMS retains information of the power consumption in a state of concealing the actual power consumption measured by the SMs and the application server calculates the total amount of the power consumption, thereby protecting a privacy. In order to let the MDMS retain the power consumptions in the state of concealing the actual power consumption, it is effective to encrypt the power consumption by each SM. In this case, a configuration which does not put much load on the SM and MDMS is required.